feat(FN-1757): clear residual lint debt in engine package

- Remove unused imports across 25 files in engine package
- Remove unused variable declarations in ipc-worker.ts, child-process-runtime.ts, and mission-autopilot.ts
- Clean up unnecessary imports in agent-instructions.ts, agent-tools.ts, cron-runner.ts, executor.ts, and other modules
- Minor cleanup in notifier.ts, peer-exchange-service.ts, pi.ts, plugin-runner.ts, and other files
- Improves code quality and reduces potential confusion from unused code

packages/engine/src/project-engine.ts

@@ -1,3 +1,4 @@
+/* eslint-disable @typescript-eslint/no-explicit-any */
 import type {
   TaskStore,
   Task,
@@ -17,7 +18,7 @@ import { CronRunner, createAiPromptExecutor } from "./cron-runner.js";
 import { aiMergeTask } from "./merger.js";
 import { PRIORITY_MERGE } from "./concurrency.js";
 import { runtimeLog } from "./logger.js";
-import type { HeartbeatMonitor, HeartbeatTriggerScheduler } from "./agent-heartbeat.js";
+import type { HeartbeatTriggerScheduler } from "./agent-heartbeat.js";
 
 /**
  * Callback for processing pull-request merge strategy.